1. Field of the Invention
The present invention is generally related to memory technology. In particular, the present invention relates to memory devices formed using chalcogenide glasses.
2. Description of the Related Art
Computers and other digital systems use memory to store programs and data. A common form of memory is random access memory (RAM). Many memory devices, such as dynamic random access memory (DRAM) devices and static random access memory (SRAM) devices are volatile memories. A volatile memory loses its data when power is removed. In addition, certain volatile memories such as DRAM devices require periodic refresh cycles to retain their data even when power is continuously supplied.
In contrast to the potential loss of data encountered in volatile memory devices, nonvolatile memory devices retain data when power is removed. Examples of nonvolatile memory devices include read only memory (ROM), programmable read only memory (PROM), erasable programmable read only memory (EPROM), electrically erasable programmable read only memory (EEPROM), and the like.
U.S. Pat. No. 6,084,796 to Kozicki, et al., entitled “Programmable metallization cell structure and method of making same,” discloses another type of nonvolatile memory device known as a programmable conductor memory cell or a programmable metallization cell (PMC). U.S. Pat. No. 6,084,796 is herein incorporated by reference in its entirety. Such memory cells can be integrated into a memory device, which is known as a programmable conductor random access memory (PCRAM). Additional applications for a programmable metallization cell include use as a programmable resistance and a programmable capacitance.
One conventional technique for producing the programmable conductor memory cell applies silver (Ag) photodoping to a chalcogenide glass, such as germanium selenide (GexSe(1−x)). As reported by Mitkova, et al, in “Dual Chemical Role of Ag as an Additive in Chalcogenide Glasses,” Physical Review Letters, Vol. 83, no. 19 (Nov. 8, 1999), pp. 3848-3851, silver (Ag) can only be photodoped into glasses of specific stoichiometries that form silver selenide and a new glass stoichiometry backbone. Further, glasses that can be photodoped with silver (Ag) are “floppy” and switch relatively slowly as compared to a rigid glass. Boolchand, et al., in Onset of Rigidity in Steps in Chalcogenide Glass, Properties and Applications of Amorphous Materials, pp. 97-132, (2001), observes a floppy to rigid transition in GexSe(1−x) glasses that occurs when x=0.23, where x corresponds to the germanium molar concentration.
In addition, Mitkova, et al., found that glasses that fall within the stoichiometry range defined by region II of FIG. 1 of the Mitkova reference do not form silver selenide when doped with silver (Ag). For example, a rigid glass, such as germanium selenide (Ge0.40Se0.60) will not form silver selenide when photodoped with silver (Ag) and, as a result, does not function as a memory switch.
The presence of silver selenide in a GexSe1−x glass photodoped with silver (Ag) allows the glass to be used as a memory switch. Glasses used for silver (Ag) incorporation via photodoping are floppy and switch more slowly electrically, and with worse memory retention, than glasses that are rigid. Preferred rigid glasses, e.g., Ge0.40S0.60, do not form silver selenide when photodoped with silver (Ag). However, the relatively fast switching times and relatively good memory retention occur in a system which incorporates silver selenide and a rigid glass such as Ge0.40Se0.60. What is needed is a technique to form this type of memory cell.